Electrolytic polishing of metals

ABSTRACT

ELECTRLYTIC POLISHING OF A METAL BODY. IN A FIRST STAGE INTERGRANULAR CORROSION OF THE BODY IS INDUCED BY ELECTROLYTIC CATHODIC TREATMENT IN AN ACIDIC ELECTROLYTE CONTAINING A COPPER SALT. IN A SECOND STAGE THE POLARITY IS REVERSED AND THE BODY IS SUBJECTED TO ELECTRLYTIC ANODIC TREATMENT. THE ELECTRLYTE IN THE SECOND STAGE MAY BE THE SAME AS IN THE FIRST STAGE OR DIFFERENT.

United States Patent Oflice' 3,829,367 Patented Aug. 13, 1974 3,829,367 ELECTROLYTIC POLISHING F METALS Samuel Raviv, 8 Deborah St., Beer-Sheba, Israel N0 Drawing. Continuation-impart of abandoned application Ser. No. 101,437, Dec. 24, 1970. This application July 14, 1972, Ser. No. 271,933

Int. Cl. C23b 3/06 US. Cl. 204-12935 10 Claims ABSTRACT OF THE DISCLOSURE Electrolytic polishing of a metal body. In a first stage intergranular corrosion of the body is induced by electrolytic cathodic treatment in an acidic electrolyte containing a copper salt. In a second stage the polarity is reversed and the body is subjected to electrolytic anodic treatment. The electrolyte in the second stage may be the same as in the first stage or different.

The present invention is a continuation-in-part of my previous application Ser. No. 101,437 now abandoned.

The present invention concerns a method for the electrolytic polishing of aluminum and its alloys, copper and its alloys and stainless steels.

It is known to polish metal surfaces electrolytically by superficial anodic dissolution of the metal. In these processes, the metal body is immersed in a suitable electrolytic bath, connected to the positive terminal of a DC. source and a second electrode is immersed in the same bath and is connected to the negative terminal of the same D.C. source, and direct electric current is allowed to flow in the circuit.

It has also been known to dissolve a metal body cathodically by immersing it in nitric acid, connecting it as cathode into an electrolytic circuit and allowing electric current to flow in the circuit until the desired degree of dissolution has occurred. This method is described in Israel Pat. No. 26,224 and corresponding foreign patents, such as French Pat. No. 1,549,365, British Pat. No. 1,129,750 and Belgian Pat. No. 704,472.

It has long been known that where a metal is com pletely immersed in an electrolyte, there occurs a deterioration of the metal which is referred to in the art as intercrystalline or intergrannular corrosion (see, for example, Thorie et Mthodes dEssai de la Corrosion des Mtaux by G. V. Akimov, N. D. Thomashov (Dunod, Paris, 1957), p. 7, 290-296). This phenomenon is generally considered as a nuisance and nobody has so far considered controlling it and utilizing it for any constructive purpose.

In accordance with the invention it has now been surprisingly discovered that intergranular corrosion of metals can be enhanced electrolytically by a controlled cathodic treatment.

It has now further been discovered in accordance with the present invention that for an effective and controlled intergranular corrosion, the formation of hydrogen should be prevented and that this is best achieved by the incorporation of a copper salt in the electrolytic solution in which the cathodic treatment is effected.

Furthermore, in accordance with the invention, there has now been made yet another surprising observation that when, subsequent to the controlled intergranular corrosion, the metal is subjected to anodic treatment, the ridges and planar stretches between the fissures that resulted from the intergranular corrosion are dissolved whereby the metal surface is smoothened and becomes highly polished with a mirror-like lustre, of a quality far surpassing that obtained by conventional polishing methods. The above applies both when the body remains in the original electrolyte and when it is transferred to a different one.

When the said anodic treatment is followed by means of profilographic measurements of the metal surface, it is observed that the degree of polish of the surface increases up to a certain time, after which it remains practically unchanged. It is believed that this stage is reached when the ridges and planar stretches are completely levelled off to become flush with the bottom of the fissures that result in the first stage from intergranular corrosion. This is, however, merely a tentative explanation which should not be construed as limiting the invention in any way.

According to the invention there is thus provided a process for the electrolytic polishing of a body made of a metal selected from the group consisting of aluminum, aluminum alloys, copper, copper alloys and stainless steels, wherein in a first stage said body is subjected for at least one minute to an electrolytic cathodic treatment in an acidic electrolyte containing a copper salt in an amount not exceeding 10 g./l. thereby to cause intergranular corrosion to occur in the surface region of said body, and in a subsequent, second stage, said body is subjected to electrolytic anodic treatment for at least one minute thereby to impart to the body a desired degree of polish, the following conditions being maintained throughout said first and second stages:

Voltage volts 5150 Current density on the body amp./dm. 1-150 Electrolytic bath temperature C 20-95 Preferably, the amount of copper salt incorporated in the electrolytic solution used for the cathodic treatment is within the range of 1-10 g./l. An example of a copper salt suitable for the purposes of the present invention is copper sulfate.

The acidic electrolytic solutions suitable for use in the method according to the invention are aqueous, but may also contain varying proportions of organic solvents, in particular lower aliphatic alcohols or polyalcohols.

The polish of the metal surface produced in accordance with the invention is of superior quality as can easily be established by the eye. The invention is thus useful in a variety of cases where high polished metal surfaces are required, e.g. in the production of metal light reflectors and mirrors, in the formation of highly polished areas on the surfaces of various metal appliances for ornamental purposes and the like.

A specific application that deserves special mentioning concerns high vacuum equipment. In such equipment the inner surfaces of all components must have a high polish in order to reduce as much as possible to gas adsorption thereon. The polish achieved by hitherto known techniques was not quite satisfactory in this respect. Against this, surfaces polished in accordance with the present invention have been found virtually not to adsorb any gas and, consequently, the techniques of the present invention can successfully be applied for polishing surfaces of high vacuum equipment.

If desired, it is possible in accordance with the invention to polish the surfaces of a metal body selectively. One way of achieving this is to immerse the body only partly in the aqueous electrolytic solution at least during the second stage. In this Way the finished body comprises two zonesa polished one and a non-polished one.

.Another way of achieving selective polishing, applicable when non-polished and polished zones are mutually interspersed, is to cover the body selectively, at least during the second stage, by insulating material not affected by the electrolytic solution. In consequence only the non-covered portions of these surfaces are polished. After the completion of the polishing the insulating material is removed. The insulating material may, for example, be a varnish or a plastic material which is applied by spraying, spreading, brushing or the like and is at the end of an operation.

3 4 In some cases it may be desirable to connect the metal 1-3. The electrolytic solutions and the conditions used in electrically prior to its immersion in the electrolyte in the process are listed in Table 2. In all cases a highly order to ensure in this way that from the outset the conpolished surface was obtained.

TABLE 2 Current '1 cath- Tempera- Voltage density, ode, T anode, turc, Ex. N0. Composition of the electrolytic bath in volts amp/dm. minutes minutes C 4 HsPO4, 85%; Propyl alcohol, 15%; 011304, 7 g./l 15 30 3 5 55 5 1131304, 70%; butyl alcohol, 30%; CuSO4, 6 g./l 3O 3 6 4O 6.. H3? 04, 65%; glycerine, 35%; 011804, 5 g./l .e 40 4 8 90 7 H31 04, 60% plus H1804, 40% to which mixture is added: CrOs, 7.5 g./l; 15 60 1 5 80 CuSO4, 4 g./l.

trolled electrolytic phenomena prevail over any possible The same solutions may also be applied for polishing purely chemical attack which might otherwise prevail at copper, aluminum and their alloys. the start.

The mechanical means by which the body is connected EXAMPLES 841 to a DC. source and retained suspended in the elec- Bodies made of pp pp alloys and pp trolytic bath are conventional and do not form part of Zine-nickel alloys, Were Submitted to 11 Procedure analothe invention. They need consequently not be described. 2011s to that described for Examples The electrolytic The following Examples illustrate the use of the method 20 s lutions and the conditions used in the process are listed according to the invention for the polishing of a number in Table 3. In all cases a highly polished surface was obof metals, it being understood that the invention is not tained.

TABLE 3 Current T cath- Tempera- Voltage density, ode, T anode, ture Ex. No. Composition of the electrolytic bath in volts amp/dm. minutes minutes C 8 HaPO, 35%; E20, 75%; 011804, 5 g./l 12 20 5 5 20 9-.." HsPO4, 60%; isopropyl alcohol, 40%; 011504, 7 g./l 12 30 2 5 30 10 H3PO4, 75%; sec.hutyl alcohol, 011804, 9 g./l 15 2 8 40 11 HsPOi, 40%; glycerine, 20%; butyl alcohol, 20%; H20, 20%; (311504, 10 g./l 15 25 2 8 40 limited to the conditions specified therein. In all the EX- The same solutions may also be used for polishing aluamples the metal bodies that were polished were first deminium, aluminum alloys and stainless steel. greased, e.g. chemically by cleansing with an organic In all the foregoing Examples 1 to 11, the metal resolvent, or electrochemically, both as known per se, and mained during both stages in the same electrolyte. Howthen rinsed with water, before being subjected to the ever, the electrolytes may also be changed in the second polishing treatment according to the invention. After comstage. For example, an aluminium alloy immersed in the pletion of the second stage of the treatment the bodies first stage in a bath of composition 1 in Table 1 may in were rinsed with water and dried. the second stage be immersed in any of the other baths indicated in the same Table. The same applies to stainless EXAMPLES 1 3 steel--Table 2, and to copper and its alloysTable 3. Bodies of aluminum and aluminium alloys were im- 1 claim: mersed in baths containing solutions having the composi- 1. A process for the electrolytic polishing of a body tions listed in the second column in Table 1 below and made of a metal selected from the group consisting of aluconnected as the cathode in a circuit in which an electric minum, aluminum alloys, copper, copper alloys and staincurrent was passed at a voltage and a current density l t l comprising indicated, respectively, in the third follrtll columns subjecting said body in a first stage for at least one minof Table 1, for a duration of time indlcated 1n the 0 111 ute to an electrolytic cathodic treatment in an acidic headed T cathode of the Table. The polarlty Was then electrolyte containing phosphoric acid and a copper reversed, while the body was kept immersed 1n the same Salt in an amount not exceeding 10 thereby bath and a current was made to flow at the same voltage and current density for the time specified in the column headed T anode of Table 1. The temperatures of the baths employed are listed in the last column of Table 1. In all Examples H PO means an aqueous solution containing 85% by weight of pure H PO and H 80 means elfecting intergranular corrosion in the surface region of said body, and

in a subsequent, second stage, subjecting said body to electrolytic anodic treatment in an acidic electrolyte containing phosphoric acid and no more than 10 g./l.

a 66 B. aqueous solution. Indications of percentage are of Copper Salt for at least one minute e y all percent v./v., and the current density is measured on P g t0 the y a deslred degree of P the object that is polished. the following conditions being maintained throughout TABLE 1 Current T cath- Tempera- Voltage density, ode, T anode, ture, Ex. No. Composition of the electrolytic bath in volts amp/(1m. minutes minutes C 1 H3P04, 55% plus H1804, 45% to which mixture is added: Cr03, 7.5 g./l.; 14 30 3 5 80 011804, 3 ./l.; H2O dist. 55 ec./l. 2.. 1131304, glycerine 40%; CuSO;, 5 g./l 15 30 2 6 60 3 Her 04, H2804, 25% to which mixture is added: HNO3,50 g./l.;CuSO4,5 14 10 2 5 In all cases a highly polished surface was obtained. The said first and second stages: same solutions as above may also be used for polishing 7 co er co er allo sand stainless steel. Voltage 5-150 pp P y Current density on thebody amp/dm. 1-150 EXAMPLES 4-7 Electrolytic bath temperature C 20-95 Bodies of stainless steel SS 304 18/8 were submitted 2. A process according to Claim 1, wherein the same to a procedure analogous to that described in Examples 75 electrolyte is used in both stages.

3. A process according to Claim 1, wherein at least during the second stage part of the body surface is covered by an insulating material so that only the non-covered portions of the surface are polished, and the insulating cover is removed after the completion of the second stage.

4. A process according to Claim 1, wherein the body is electrically connected prior to its immersion in the electrolytic solution.

5. A process according to Claim 1, wherein the electrolyte is an aqueous solution of phosphoric acid, sulfuric acid, chromic acid and 1-10 g./l. of a copper salt.

6. A process according to Claim 1, wherein the electrolyte is an aqueous solution containing phosphoric acid, nitric acid, sulfuric acid and 1-10 g./l. of a copper salt.

7. A process according to Claim 1, wherein the electrolyte is an aqueous solution containing phosphoric acid, at least one alcohol and 1-10 g./l. of a copper salt.

8. A process according to Claim 7, wherein the alcohol is propyl alcohol, butyl alcohol or glycerine.

References Cited UNITED STATES PATENTS 3,287,238 11/1966 Latawiec 204129.35 3,519,779 7/1970 Dolomont 204129.9 3,389,065 6/1968 Shibasaki 204129.8 2,366,714 1/1945 Faust 204-129.9 2,706,171 4/1955 Goral 204-129.9

THOMAS M. TUFARIELLO, Primary Examiner US. Cl. X.R. 204-l29 

